1. Field of the Invention
The present invention relates to a unit of light emitting diode arrays. More particularly, the present invention relates to a unit of light emitting diode arrays to be used in an optical print head and the like, in which a large number of light emitting arrays are provided on a substrate.
2. Description of the Prior Art
In the prior art, U.S. Pat. No. 3,850,517 to Joseph F. Stephany, "High Speed Printout System" is known as an optical printer using light emitting diode arrays as a light source. As light emitting diode arrays to be used in such an optical printer, Japanese Patent Laying-Open Gazette No. 125878/1983, "Light Emitting Device Fixing Substrate" invented by Takeshi Mizutani et al. and Japanese Patent Laying-Open Gazette No. 22372/1984, "Semiconductor Light Emitting Device" invented by Tsutomu Koshimura are known.
FIG. 1 is a perspective appearance view showing an example of a conventional light emitting device in which the "Light Emitting Device Fixing Substrate" of the above stated Japanese patent Laying-Open Gazette No. 125878/1983 is applied.
First, referring to FIG. 1, a light emitting device 1 to be used in a conventional optical printer will be described. The light emitting device 1 comprises a substrate 2 and a large number of light emitting diode arrays 4 having a large number of light emitting areas 3 provided on the substrate 2. Electrodes 5 are formed corresponding to the respective light emitting diode arrays 4 and each electrode 5 is connected through a metallic fine wire 6 to a fine pattern 7 formed in the vicinity of the light emitting diode arrays 4.
FIG. 2 is a sectional view of an essential portion of a conventional light emitting diode array and FIG. 3 is a plane view of the essential portion.
Each of the light emitting diode arrays 4 shown in FIG. 1 includes a P type region 10 formed by selectively diffusing a P type dopant using a mask, an insulating film 8 having an opening 81 on an n type layer 9 of GaAsP and an electrode 5 of aluminum is formed to cover partially the P type region 10.
The electrode 5 has a top end 51 of a width equal to the length of one side of the opening 81 of the insulating film 8, as shown in FIG. 3. Thus, since the electrode 5 can be formed by using one side of the opening 81 of the insulating film 8 as a reference line, positioning of a mask having an electrode pattern can be made easily at the time of evaporation of the electrode 5.
Therefore, a method to form an electrode 51 having a top end of the same width as that of a P type region 10 as shown in FIG. 3 may be considered. This method makes it possible to set a mask for electrodes by using the side edges of the P type region 10 as reference lines and thus, positioning can be made easily.
However, if a P type region 10 assigned for a light emitting region is formed by diffusion on the n type layer 9 as show in FIG. 2, the diffused layer, that is, the P type region 10 becomes wider as it comes up to the surface of the n type layer 9, and thus, this P type region 10 has a sectional shape like a saucer due to such enhanced diffusion. Further, light from the P type region 10 is emitted not only perpendicularly to the surface but also radially. Accordingly, if electric current is supplied to the Pn junction to emit light, expansion of the P type region 10 due to such spreading diffusion and enlargement of the luminous flux cause light to be emitted with an area larger than the area of the opening 81 of the insulating film 8 by 2 to 10% as a whole.
On the other hand, since the electrode 5 of aluminum has naturally the light intercepting property, the light emitted from the surface of the P type region 10 located under the electrode 5 is intercepted. However, the light from the portions spread by the diffusion leaks from both sides of the electrode 51 and as a result, the dot shape corresponds to a printing dot in the shape almost like the letter U as shown by the shaded portion in FIG. 3. Since the depth of the P type region 10 is as shallow as 5.0 .mu.m (generally, approximately 2.0 .mu.m), the rays of light around the electrode 51 do not cause any serious influence when the density of printing dots is low (for example, the density is 3.85 dots/mm and the width of a printing dot is less than 50% of a spacing of dots). However, if the density of printing dots is high (for example, 9.5 dots/mm or more), the rays of light around the electrode 51 become conspicuously bright and particularly the Pn junction under the electrode 51 is liable to have high brightness since electric current tends to concentrate in the Pn junction. In consequence, a drawback is involved that if electric current higher than 8 mA/dot flows, the printing dots have a shape nearly coincident to the letter U and thus, the printing quality is deteriorated.
On the other hand, the above mentioned Japanese Patent Laying-Open Gazette No. 22372/1984, "Semiconductor Light Emitting Device" discloses the use of an electrode having a top end in the shape of the letter U so as to cover at least three sides of a P type region. Thus, the above stated rays of light can be intercepted by making an electrode surround the P type region or making a sufficiently wide electrode cover one side of the P type region. However, such shape of an electrode makes it difficult to arrange a high density of electrodes. More specifically, positioning of a mask for forming electrodes cannot be made easily and deviation is liable to occur in the position of the electrode pattern. As a result, an electrode in a P type region comes too close to the adjacent P type region or adjacent electrode, which often causes erroneous emission of light or a short circuit due to leakage of electric current. Accordingly, there is a disadvantage that the printed picture quality is rough since P type regions for forming printing dots must be disposed with a sufficient spacing.